docs: link to the developer wiki documentation

Signed-off-by: Karl Palsson <karlp@etactica.com>

README.md

@@ -21,6 +21,10 @@ To install all its package definitions, run:
 
 You can browse the generated API documentation [directly on Github](http://htmlpreview.github.io/?http://raw.githubusercontent.com/openwrt/luci/master/documentation/api/index.html).
 
+## Development
+
+Documentation for developing and extending LuCI can be found [in the Wiki](https://github.com/openwrt/luci/wiki)
+
 ## License
 
 See [LICENSE](LICENSE) file.